Potential for spider mites, especially in southern Michigan

The upcoming dry weather may set up a potential outbreak of spider mites. Judging the need to treat an infested field can be difficult. A descriptive rating system can help you make a treatment decision.

Southern and central Michigan are expected to remain dry into July, and this sets up a potential outbreak for spider mites. A rating system can help you decide if treatment is needed. A bulletin describing the rating system, with pictures, is available at Scouting and Managing Spider Mite in Soybean.

Mites move into fields from the edge, often by passive airborne movement. Damage may be first noticed near an obstacle, such as a tree line or power pole, that disrupts wind near the edge of the field. A mite feeds with a piercing mouthpart that it inserts directly into plant cells, sucking out the contents. This results in tiny, yellow dots or specks on the leaves. As mite numbers increase, the yellowing becomes more apparent across the leaf surface. Cells are killed and water is lost through the wounds.

Under severe infestation, whole leaves turn brown and eventually drop off the plant. Under a low infestation, you may need a hand lens to find mites on leaves, or better yet, shake plant foliage over a white piece of paper. A heavy mite infestation, however, is fairly obvious to the naked eye, with leaf yellowing, huge numbers of mites on the undersides of leaves, plus webbing. Treatment options are insecticides with the OPs chlorpyrifos, dimethoate (Lorsban, Dimethoate, Cobalt) or bifenthrin (such as Capture, Hero).

Unfortunately, if you treat for spider mites, populations can resurge due to:

  • Egg hatch. Mites lay eggs on the plant surface. Insecticides kill adults and nymphs, but do not kill eggs. Since Dimethoate and Lorsban have a short residual, newly hatched nymphs survive and repopulate the plants.
  • Rebound or flaring. Insecticides kill beneficial insects, but don’t kill 100 percent of the mites. The mites reproduce in the absence of predators, leading to a rapid increase (flaring) of the population. This is why we recommend scouting and spraying only when mites reach a threshold, avoiding insurance applications of insecticide for mites as well as soybean aphids. Also, be careful about fungicide applications (see The hidden costs of insurance pesticide applications).
  • Resistance. Spraying can quickly shift a spider mite population to resistant individuals. This problem increases with the number of applications. This is another reason we recommend scouting and spraying only when mites have reached a threshold.

If you do plan to treat, check fields before you spray to make sure mites are still present, as populations can crash quickly. Rain itself reduces plant stress and replaces water lost to pest feeding. But more importantly, high humidity is critical for promoting the growth of fungi that naturally infest and kill mites. Humidity must be elevated for an extended time, 48 hours or more, before naturally occurring fungi are active. Mite populations can crash in a matter of days once fungal pathogens become active.

Dr. DiFonzo’s work is funded in part by MSU‘s AgBioResearch.

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